French patent application number No. 2 593 761 for "A railway signalling system for recognizing the passage of a predetermined vehicle past a given point" describes a system including a fixed interrogation beacon and a passive responder fixed on the vehicle to be recognized. The beacon emits a wave modulated at a first frequency and receives a wave returned by the responder in which modulation is detected at a second frequency lower than the first frequency. The responder has no internal power source and comprises a microwave receiver, a divider for dividing the energy it receives into two equal portions, a detector, a delay line, a modulator, and a transmission antenna designed to split up the received modulated wave into pulses of width equal to the time constant of the delay line. In order to recognize different types of vehicle, responders are provided having delay lines with different time constants, and the beacon requires as many delay lines as there are types of vehicle. By detecting the modulated wave transmitted by the transponder, it is possible to recognize the time constant of the delay line fitted to the responder, and thus to determine the type of vehicle. The responder thus transmits a single signal having a special characteristic and the information which can be conveyed by such a signal is necessarily limited. In addition, the structure of the responder is relatively complicated and requires several components.
U.S. patent application Ser. No. 131,796, filed by the present Applicants on Dec. 11, 1987, for "A device for wide band transmission of information and/or instructions between a rail vehicle and a traffic control station" describes a device in which information is transmitted by means of a waveguide disposed along the track. The emitting face of the waveguide is pierced by a network of openings, with some of the openings extending perpendicularly to the longitudinal axis of the waveguide and other openings extending obliquely relative to said axis, and being disposed in a special pattern corresponding to an appropriate code. The openings extending perpendicularly to the axis transmit an axial component Ez conveying information and/or instructions, and the oblique openings transmit an additional component Ey perpendicular to the axis enabling the vehicle to determine its absolute position and any other information related thereto, in particular a maximum speed limit. The waveguide is connected to a ground station provided with transmission and reception means. The vehicle is equipped with at least one antenna for transmission and/or reception, together with transmitter and/or receiver means. The ground station connected to the waveguide transmits two microwaves at different frequencies, one of which is used for interchanging information and/or instructions, and the other of which gives rise to large amplitude fluctuations in the signals received by the antenna fixed to the vehicle and remaining in the proximity of the face of the waveguide through which the openings are pierced, so as to enable the position of the vehicle to be determined by counting the numbers of oblique openings, and thus enabling the speed of the vehicle to be measured.
This system requires a power supply both for the transmitter and for the receiver.
When identification transponders are disposed at certain points along the track, these transponders need to be equipped with self-contained power supplies, e.g. based on batteries, and even when long duration batteries are used this means that the batteries need to be checked periodically. In addition, such transponders cease to operate at low temperatures since the batteries can no longer provide enough energy.
The object of the present invention is to provide an identification transponder which is passive and which therefore does not require any kind of electrical power supply.